1. Field of the Invention
The present invention relates to a dielectric filter comprising a plurality of dielectric resonators arranged in parallel with each other.
2. Prior Art
There are known dielectric filters of the type comprising a rectangularly parallelepipedic dielectric ceramic block, three or more than three resonators provided by boring so many through holes in the dielectric ceramic block and coating the peripheral walls thereof with an inner conductor and an outer conductor covering the outer peripheral surface of the dielectric ceramic block, excepting one end surface of the dielectric ceramic block which forms an open circuit end on which one opening ends of the through holes are positioned.
FIGS. 1 and 2 of the accompanying drawings show a conventional dielectric filter of the above identified type with three resonators A and B, wherein a pair of input/output pads P are provided on a lateral side surface of the dielectric ceramic block, arranged at locations close to the open circuit end surface of the dielectric ceramic block and facing the respective outer resonators B and capacitively coupled to the respective outer resonators B, each of said input/output pads P being separated from the outer conductor C by an insulating zone D surrounding it. Such conventional dielectric filter is disclosed in U.S. Pat. No. 5,146,193.
While each of the resonators A and B is normally made to have a length equal to .lambda./4 or a quarter of the specified resonant frequency, the above arrangement of input/output pads P disposed vis-a-vis the respective outer resonators B at locations close to the open circuit end surface gives rise to a problem that the resonant frequency of the outer resonators B is increased due to the provision of the input/output pads P separated from the outer conductor C by respective insulating zones D and the partial removal of the outer conductor and consequently the resonant frequency of the outer resonators comes to disagree with that of the central resonator A at the cost of filtering performance.
This problem may be dissolved by forming a recess E in a central area of the bottom or short circuit end surface of the dielectric ceramic block as shown in FIGS. 1 and 2 to make the resonant length of the outer resonators B longer than that of the central resonator A and shift the resonant frequency of the resonators B downward in advance in order to compensate the raised resonant frequency of the outer resonators B caused by the input/output pads P and make the resonant frequencies of all the resonators A and B consequently agree with each other. FIG. 2 of Japanese Patent Kokai No. 62-104201 discloses one example of this frequency compensating method.
It should be noted, however, that a conductive film F is formed on the short circuit end surface of the dielectric ceramic block in such known dielectric filter and connected to the edges of the other opening ends of the resonators on the short circuit end surface. The conductive film F is typically prepared by a screen printing which is adapted to mass production. However, with the configuration of the dielectric filter of FIGS. 1 and 2 having a recess E formed in a central area of the short circuit end, the screen printing technique cannot feasibly be used and the conductive film F has to be formed by applying a conductive material to that side by means of a brush at the cost of manufacturing efficiency. In short, such a configuration is not adapted to mass production.
It is therefore an object of the present invention to provide a dielectric filter that can make the resonant frequencies of the resonators agree with each other and, at the same time, capacitively couple the input/output pads and the respective outermost resonators without requiring the formation of a recess on the short-circuiting end of a dielectric ceramic block.